Optimizing away empty loop with step

[Vladimir Panteleev]

DMD [fails](https://issues.dlang.org/show_bug.cgi?id=22158) to 
optimize away an empty for loop with an increment of 1. GDC and 
LDC both succeed.

However, the results are more interesting if you make the step 
variable:

```d
void fun(int a, int b, int step)
{
     for (int i = a; i < b; i += step) {
     }
}
```

The above code (valid C and D) compiles to an empty function with 
clang and gcc:

https://godbolt.org/z/PnffvhecM

However, all three D compilers fail to optimize away the loop:

https://d.godbolt.org/z/r3MEGzoh1

Is the frontend doing something to prevent the loop from being 
optimized away as in C?

My first guess was that the compiler refused to optimize it away 
because it couldn't prove that the loop will ever exit. But, the 
C compilers don't seem to have an issue with that.

My second guess was that the C compilers were happy with 
optimizing it away because signed integer overflow is undefined 
in C. But, changing the type to unsigned didn't make a difference.

https://godbolt.org/z/hMb83ne9d